An earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by rotating the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. With weight applied to the drill string, the rotating drill bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone. The borehole formed in the drilling process will have a diameter generally equal to the diameter or "gage" of the drill bit.
A typical earth-boring bit includes one or more rotatable cutters that perform their cutting function due to the rolling movement of the cutters acting against the formation material. The cutters roll and slide upon the bottom of the borehole as the bit is rotated, the cutters thereby engaging and disintegrating the formation material in its path. The rotatable cutters may be described as generally conical in shape and are therefore sometimes referred to as rolling cones. Such bits typically include a bit body with a plurality of journal segment legs. The cutters are mounted on bearing pin shafts that extend downwardly and inwardly from the journal segment legs. The borehole is formed as the gouging and scraping or crushing and chipping action of the rotary cones remove chips of formation material which are carried upward and out of the borehole by drilling fluid which is pumped downwardly through the drill pipe and out of the bit.
The earth disintegrating action of the rolling cone cutters is enhanced by providing the cutters with a plurality of cutter elements. Cutter elements are generally of two types: inserts formed of a very hard material, such as cemented tungsten carbide, that are press fit into undersized apertures in the cone surface; or teeth that are milled, cast or otherwise integrally formed from the material of the rolling cone. Bits having cemented tungsten carbide inserts are typically referred to as "TCI" bits, while those having teeth formed from the cone material are known as "steel tooth bits." The cutting surfaces of inserts are, in some instances, coated with a very hard "superabrasive" coating such as polycrystalline diamond (PCD) or cubic boron nitride (PCBN). Superabrasive materials are significantly harder than cemented tungsten carbide. As used herein, the term "superabrasive" means a material having a hardness of at least 2,700 Knoop (kg/mm.sup.2). PCD grades have a hardness range of about 5,000-8,000 Knoop, while PCBN grades have a hardness range of about 2,700-3,500 Knoop. By way of comparison, a typical cemented tungsten carbide grade used to form cutter elements has a hardness of about 1475 Knoop.
Similarly, the teeth of steel tooth bits may be coated with a hard metal layer generally referred to as hardfacing. In each case, the cutter elements on the rotating cutters functionally breakup the formation to create new borehole by a combination of gouging and scraping or chipping and crushing.
The cost of drilling a borehole is proportional to the length of time it takes to drill to the desired depth and location. In oil and gas drilling, the time required to drill the well, in turn, is greatly affected by the number of times the drill bit must be changed in order to reach the targeted formation. This is the case because each time the bit is changed, the entire string of drill pipe, which may be miles long, must be retrieved from the borehole, section by section. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which again must be constructed section by section. As is thus obvious, this process, known as a "trip" of the drill string, requires considerable time, effort and expense. Accordingly, it is always desirable to employ drill bits which will drill faster and longer and which are usable over a wider range of formation hardness.
The length of time that a drill bit may be employed before it must be changed depends upon its rate of penetration ("ROP"), as well as its durability or ability to maintain an acceptable ROP. As is apparent, dull, broken or worn cutter elements cause a decrease in ROP. The form and positioning of the cutter elements (both steel teeth and TCI inserts) upon the cutters greatly impact bit durability and ROP and thus are critical to the success of a particular bit design.
Bit durability is, in part, measured by a bit's ability to "hold gage," meaning its ability to maintain a full gage borehole diameter over the entire length of the borehole. Gage holding ability is particularly vital in directional drilling applications, which have become increasingly important. If gage is not maintained at a relatively constant dimension, it becomes more difficult, and thus more costly, to insert drilling apparatus into the borehole than if the borehole had a constant diameter. For example, when a new, unworn bit is inserted into an undergage borehole, the new bit will be required to ream the undergage hole as it progresses toward the bottom of the borehole. Thus, by the time it reaches the bottom, the bit may have experienced a substantial amount of wear that it would not have experienced had the prior bit been able to maintain full gage. This unnecessary wear will shorten the bit life of the newly-inserted bit, thus prematurely requiring the time consuming and expensive process of removing the drill string, replacing the worn bit, and reinstalling another new bit downhole.
To assist in maintaining the gage of a borehole, conventional rolling cone bits typically employ a heel row of hard metal inserts on the heel surface of the rolling cone cutters. The heel surface is a generally frustoconical surface and is configured and positioned so as to generally align with and ream the sidewall of the borehole as the bit rotates. The inserts in the heel surface contact the borehole wall with a sliding motion and thus generally may be described as scraping or reaming the borehole sidewall. The heel inserts function primarily to maintain a constant gage and secondarily to prevent the erosion and abrasion of the heel surface of the rolling cone. Excessive wear of the heel inserts leads to an undergage borehole, decreased ROP and increased loading on the other cutter elements on the bit. It may also accelerate wear of the cutter bearing and ultimately lead to bit failure.
In addition to the heel row inserts, conventional bits typically include a gage row of cutter elements mounted adjacent to the heel surface but oriented and sized in such a manner so as to cut the corner of the borehole. Conventional bits also include a number of additional rows of cutter elements that are located on the cones in rows disposed radially inward from the gage row. These cutter elements are sized and configured for cutting the bottom of the borehole and are typically described as inner row cutter elements.
Each cutter element on the bit has what is termed a leading face or edge and a trailing face or edge. The leading face or edge is defined as that portion of the cutting surface of the cutter element that first contacts the formation as the bit rotates. The trailing face or edge is the portion of the cutter opposite the leading face or edge. The terms "leading" and "trailing" will be used hereinafter to refer to these portions respectively, regardless of whether the section so referred to is planar, contoured or includes an edge. Because the precise portion of the cutter element meeting each definition varies not only with bit design and cutter element design, but also with movement of the rolling cone, it will be understood by those skilled in the art that the terms "leading" and "trailing" are functional and are each meant to be defined in terms of the operation of the drill bit and cutter element itself.
It has been found that the trailing section of each cutter element is generally subject to earlier failure than the leading section. It is believed that premature failure of the trailing section, and ultimately of the whole cutter, is the result of excessive friction along the trailing section and of the resultant tensile stresses in the direction of cutting movement. Unlike the leading section, the trailing section of the cutter does not engage in shearing or reaming of the borehole wall and is not subject to large compressive stresses in the direction of cutting movement. Inserts coated with superabrasive materials, such as PDC and PCBN, are also adversely affected by the same resultant tensile stress. Because diamond is relatively brittle, diamond coating tends to crack and break off, leaving the insert unprotected. Diamond coated inserts are better suited to withstand wear and frictional heat compared to uncoated inserts.